1. Field of the Invention
The present invention relates to a screenprinting device having a fabric and a template situated in the fabric in the form of a photolithographically structured emulsion. The present invention also relates to a method for producing a screenprinting device of this type.
2. Description of the Related Art
Screenprinting is a printing method in which a printing ink or printing paste is pressed using a knife-like tool, the rubber squeegee (printing squeegee), through a fine-meshed fabric onto the material to be printed. Screenprinting is therefore also referred to as the through printing method. At the point of the fabric at which no ink is to be printed in accordance with the motif, the mesh openings of the fabric are impermeable to the printing ink or printing paste due to a template (e.g., photolithographically structured emulsion) situated on the fabric.
In addition to use in the field of advertisement and inscription, and in textile or ceramic printing, screenprinting is currently also frequently employed for printing circuits in the field of hybrid technology, for example, in the field of multilayer ceramic technology. An example of a multilayer ceramic technology is the so-called LTCC technology (LTCC=Low Temperature Cofired Ceramics), which represents a cost-effective technology for producing multilayer circuits on the basis of sintered ceramic carriers, which contain wiring levels connected by z contacts, so-called vias, in multiple layers. In LTCC technology, the circuit elements are applied using screenprinting to the green films of the later ceramic carrier, which are then stacked and sintered.
Modern packaging development currently demands the printing of finer structures (ultrafine line structures), regardless of the area of use, to save installation space or to minimize the consumption of high-cost pastes. In addition, modern high-frequency technology requires narrow printed conductor widths as a function of the usage frequency, which are predefined by extensive simulations based on losses and given impedances. It is therefore desirable to print finer structures. In addition, if it is possible to print finer structures, multilayer technology processes such as LTCC may replace circuits which have been produced up to this point using thin-film technology. Thin-film technology has been used up to this point in the area of high-frequency circuits in the ultrahigh frequency range to implement HF-capable structures because of its high structural resolution. This technology is implemented by deposition and etching procedures. It requires the use of very flat, pretreated, and high-cost substrates. In addition, the thin-film process per se is a costly method. Thin-film structures may additionally only be implemented in a coplanar manner on the substrate surface. The use of the multilayer technology having ultrafine line structures may provide a significant cost reduction in relation to thin-film technology and additionally offer the advantage of the use of multiple levels, inter alia, also for shield layers.
For screenprinting in the field of multilayer technology (thick-film technology), the screenprinting frame is usually made of aluminum and is covered by a steel fabric, using which the elastic deflection of the screen required during the printing procedure may be achieved. An elastic deflection of the screen during the printing procedure is necessary for the so-called lift-off, i.e., for the distance which may be implemented between fabric and substrate to be printed. Too little lift-off may result in cloudiness in the print, for example, because the fabric does not immediately detach from the printed paste film behind the squeegee—it remains “stuck” in the printed paste. Too much lift-off, in contrast, increases the fabric tension, which on one hand results in the elastic proof stress of the fabric being exceeded and thus the fabric aging prematurely, and, on the other hand, may result in blotted prints because of paste spray, so that the template edge may no longer draw a clean printed image.
The wire thickness of the fabric used is currently between approximately 30 μm and 16 μm. The permeability of the fabric is described by its mesh width, which is specified using the so-called mesh count. For example, 325 mesh means that there are 325 meshes per square inch.
The template is frequently produced as a direct template using a photographic method. For this purpose, the fabric is coated using photosensitive polymers, which are exposed using the desired structures. Subsequently, the exposed structures are developed and the unexposed areas are washed out. The fabric, the template (emulsion), and the printing frame together form the screenprinting screen.
During printing, the printing paste is applied to the screen and distributed uniformly onto the structured screen using a so-called flood bar. Subsequently, the actual printing procedure is performed, the printing squeegee being drawn over the screen using an appropriately tailored hardness. The screen is located at a specific distance from the substrate to be printed, such as an LTCC film, during this printing procedure. The screen is pressed elastically downward in the direction of the substrate to be printed using the printing squeegee. Shearing of the printing paste occurs simultaneously using the printing squeegee, which reduces its viscosity during the shearing because of its thixotropic property and may thus be pressed through the openings of the screenprinting screen. After the shear strain is ended, the printing paste has the starting viscosity again.
If smaller resolutions of the printed structures (ultrafine line structures) are to be achieved, i.e., a resolution less than 50 μm or even less than 30 μm, the problem results that for this purpose, the fabric and the template must accordingly have fine structures having small openings and these fine structures and small openings in the template and the fabric inhibit the ink or paste flow through the screenprinting screen.
The problem of increasing the register accuracy during screenprinting is solved in DE 197 38 873 A1. Moreover, the publication concerns itself with the question of optimizing the printing quality with fine strokes and rasters for plastic fabric. The plastic threads of the fabric are coated by a mantle layer which is vapor deposited or sputtered on, and which is in turn covered by a metal topcoat, which carries the emulsion of the template and results through galvanization. The mantle layer is generated using a vapor deposition or sputtering process having a layer thickness of approximately 5 nm to greater than 200 nm. The application of the mantle layer is performed using galvanic deposition. For example, a copper or nickel layer is applied. The metal-plated plastic fabric causes a highly reproducible template quality having excellent boundary sharpness and exact color metering, because it ensures extremely minimal stretching with sufficient basic consistency. The fabric known from this publication thus does not solve the problem specified above, because it relates to a plastic fabric and not to a steel fabric, which is used for printing circuit elements. In addition, the production method for a screenprinting device specified in the publication DE 197 38 873 A1 is very complex and costly.
The publication DE 10 2004 055 113 A1 discloses a method for hydrophilizing the screenprinting template carriers, which significantly improves the wetting of the screenprinting template carrier with template material. During the hydrophilizing of the screenprinting template carrier, i.e., the screenprinting fabric, it is provided with ultra-fine divided oxide particles, such as nanometer particles made of metal oxide, for example, titanium oxide, aluminum oxide, or zirconium oxide, and a wetting agent. For example, a surfactant may be used as the wetting agent. Alternatively thereto, the hydrophilizing agent may also be used during the removal of template material from the screenprinting fabric, preferably in that it is added to the layer removal liquid. During the layer removal of the screenprinting template carrier, in which it is prepared for the production of a new screenprinting screen having a new template, the screenprinting template carrier is not only freed of the template material, but rather simultaneously also hydrophilized for the next coating procedure. Therefore, the coating of the screenprinting fabric specified in this publication also does not solve the problem disclosed above of generating finer structures.